The present invention relates to converting analog information to digital information. More particularly, the present invention relates to high-speed comparators that will provide a low level or a high level indication based upon an input signal level.
Analog to digital conversions are often implemented using quantizers that sample the input analog signal and output a high or low voltage depending upon the signal level of the input signal. Quantizers for analog to digital conversions with high frequency input signals must be capable of making decisions quickly and reliably. These high input frequencies and associated high frequency sampling signals create significant problems in achieving these goals of speed and reliability.
Conventional quantizers rely upon inverter circuits to provide a positive-gain high or low level decision. The performance of such typical inverter circuits, however, suffer at high-frequencies, for example, at frequencies above about 1 GHz.
In accordance with the present invention, circuitry and an associated method are disclosed that provide a high-speed comparator. The comparator of the present invention utilizes input circuitry to receive the input signal and resonant tunneling diode (RTD) circuitry to provide the high or low level determination. The RTD circuitry may be made weak compared to the input circuitry to eliminate hysteresis, and the comparators may be cascaded together to provide a large positive-gain. In addition, clocked switches may be added to the cascaded comparator circuitry to create a clocked quantizer for analog to digital conversion.
In one embodiment, the present invention is a comparator including input circuitry having an input signal node, a reference signal node, and an output signal node, and resonant tunneling diode circuitry coupled to the output signal node. The input circuitry and the resonant tunneling diode circuitry work together to provide at the output signal node a stable low voltage point and a stable high voltage point depending upon the level of the input signal applied to the input signal node. In a further embodiment, the resonant tunneling diode circuitry has a weaker current level than the current level for the input circuitry. Still further, the current level for the resonant tunneling diode circuitry is at least about 10 times less than the current level for the input circuitry.
In more detailed respects, the resonant tunneling diode circuitry may include a pair of resonant tunneling diodes. In another more detailed embodiment, the input circuitry includes a first MOS transistor connected in a source-follower configuration with a gate of the first MOS transistor being coupled to the input signal node, and a second MOS transistor connected as a current sink with a gate node of the second MOS transistor being coupled to the reference voltage node.
In a further embodiment, the present invention is comparator circuitry including a plurality of the above comparators cascaded together such that the output signal node of a previous comparator is connected to the input signal node of the next comparator.
In a still further embodiment, the present invention is a clocked quantizer including an input signal node, an output signal node, the above cascaded comparator circuitry, and a switch coupled between the input signal node and the cascaded comparator circuitry to control when the clocked quantizer samples a signal applied to the input signal node. The switch is controlled by a clock signal. In addition, the clocked quantizer may include a second switch coupled between the output and the input of the comparator circuit to control when the output signal at the output signal node is latched. The second switch is controlled by a second clock signal. Still further, in the clocked quantizer may include a second comparator circuit coupled between the switch and the input signal node.
In another respect, the present invention is a method for providing a low level or a high level indication based upon the level on an input signal. This method includes receiving an input signal, driving the input signal with input circuitry to produce an output signal, and utilizing resonant tunneling diode circuitry to produce for the output signal a stable low voltage point or a stable high voltage point depending upon the level of the input signal. In a more detailed embodiment, the resonant tunneling diode circuitry has a weaker current level than the current level for the input circuitry so that hysteresis is reduced or eliminated. In addition, the resonant tunneling diode circuitry may include a first resonant tunneling diode coupled between the output signal node and a first voltage node and a second resonant tunneling diode coupled between the output signal node and a second voltage node.
In another embodiment, the method of the present invention may include repeating the driving and utilizing steps a plurality of times by cascading together a plurality of comparators, with each comparator including input circuitry and resonant tunneling diode circuitry. The method may also include providing an overall gain that is much greater than one. Finally, after the receiving step, the method may include controlling when the input signal is provided to the plurality of comparators so that the input signal is sampled at a desired rate.